Description of prior art monotonic DAC can be found in H. J. Schouwenaars et al., “A low-power stereo 16-bit CMOS D/A converter for digital audio”, IEEE J. Solid-State Circuits, vol. 23, no. 6, pp. 1290–1297, December 1988.
A basic diagram of a segmented current dividing DAC is shown in FIG. 1. An array of C linearly weighted coarse current sources is used. To increase the resolution, one of the current sources, in the case presented in FIG. 1 current source Ic, can be divided into more fine levels by a passive current divider. Depending on the value of the data signal, a number of the currents are switched to the output terminal Iout, and the remaining currents are dumped to signal ground.
To guarantee monotonicity of such a DAC, a principle as shown in FIG. 2 may be used. The coarse current sources are connected to three way switches. A decoder transforms the binary information of the most significant bits of the digital input signal into a thermometer code, which controls the coarse current switches. The first unselected coarse current source is connected by the three way switches to a fine bit divider stage, which divides the applied current into binary weighted current levels. These binary currents are switched to the output line by two-way switches controlled by the least significant bits. The coarse and fine output currents are added, thus forming an output current signal which is converted to an output voltage which is an analog equivalent of the digital input signal.
EP 0 359 315 discloses a digital-to-analog converter comprising a current source arrangement. The current source arrangement comprises a number of current sources and a switching network. The most significant bits of a digital input are sent to a first block comprising a decoding device, a current source for generating substantially equal currents and a switching network comprising two-way switches. A current coming from the first block is then sent to the second block comprising a current dividing circuit and a switching network. The current dividing circuit supplies the currents for the 10 least significant bits. The 10 least significant bits directly control the switches of the switching network.
The above described DAC's have a voltage output. If it is desired to obtain a current output, then the amplifier converting the current output to a voltage output may be left out. However, when doing this and when working in a wide range of output voltages, the current output is not precise due to a changing voltage at the outputs of coarse and fine blocks.
In U.S. Pat. No 4,292,625 a monotonic digital-to-analog converter is described for converting a binary number in an analog signal. The converter comprises a segment generator and a step generator. The segment generator is disposed for providing a first signal proportional to the values of the most significant digits of the binary numbers. Further, a step generator is disposed for providing a second signal proportional to the values of the least significant digits of the same binary numbers. Additionally, means for combining the first and second signals is provided to form an analog signal proportional to the value of the binary number to be converted. The output of the converter is a current.